Tropical Cyclones
A tropical cyclone is a cell of very low pressure with a steep pressure gradient, strong winds
and heavy rain
Naming:
• Hurricanes – North America and the Caribbean
• Typhoons – Asia
• Tropical Cyclones – Australia, east coast of Southern Africa, Madagascar
General Characteristics
• Forward movement – 20-25 km/h – east to west
• Wind speed of winds circulating in system – 200-300 km/h – very steep pressure
gradient
• Pressure in the eye – 940 hectopascals (hPa) – area of lowest pressure
• Diameter of eye – 30-50 km – surrounded by vortices comprised of cumulonimbus
clouds
• Pattern of isobars – roughly circular
• Vertical extent of cloud wall – 15km high
• Diameter of storm – 500km
Factors necessary for formation
1. Tropical ocean water temperatures of at least 27°C
, - Convection currents as air rises – LP develops
- Evaporation
- Provide thermal energy
2. Coriolis Force
- Air can’t blow across isobars from HP to LP – tropical cyclone will fill up
- Air must blow parallel to isobars – PGF and CF are balanced – geostrophic flow
- Causes rotating winds – pressure continues to drop
3. Stretch of open ocean – not land
- Warm water releases latent heat from rising and condensation – powers tropical
cyclone
- Warm, moist, rising air is unstable – allows for further uplift
- Minimal friction
4. Light wind shear
- System stays intact – clouds not blown away
- Strong winds – prevent formation of vortex
5. High humidity
- Latent heat (heat stored when evaporation takes place) is released after
condensation and provides thermal energy
6. Unstable air
- Air must rise so pressure drops
- Rising air cools – condensation – releases latent heat
7. Very low LP and steep pressure gradient
- Rapid convergence – air starts to rise
- Steep pressure gradient – strengthens CF – causes air to rotate
8. Divergence in upper air
- Removes air at higher altitudes and maintains LP on surface
9. No friction
- Winds can reach great speeds – CF remains strong – rotation continues
Storm levels Kilometres per Hour Knots
(kph)
Tropical depression 0 - 60 0 - 33
Group of storms that gets
organised and starts to circulate
A tropical cyclone is a cell of very low pressure with a steep pressure gradient, strong winds
and heavy rain
Naming:
• Hurricanes – North America and the Caribbean
• Typhoons – Asia
• Tropical Cyclones – Australia, east coast of Southern Africa, Madagascar
General Characteristics
• Forward movement – 20-25 km/h – east to west
• Wind speed of winds circulating in system – 200-300 km/h – very steep pressure
gradient
• Pressure in the eye – 940 hectopascals (hPa) – area of lowest pressure
• Diameter of eye – 30-50 km – surrounded by vortices comprised of cumulonimbus
clouds
• Pattern of isobars – roughly circular
• Vertical extent of cloud wall – 15km high
• Diameter of storm – 500km
Factors necessary for formation
1. Tropical ocean water temperatures of at least 27°C
, - Convection currents as air rises – LP develops
- Evaporation
- Provide thermal energy
2. Coriolis Force
- Air can’t blow across isobars from HP to LP – tropical cyclone will fill up
- Air must blow parallel to isobars – PGF and CF are balanced – geostrophic flow
- Causes rotating winds – pressure continues to drop
3. Stretch of open ocean – not land
- Warm water releases latent heat from rising and condensation – powers tropical
cyclone
- Warm, moist, rising air is unstable – allows for further uplift
- Minimal friction
4. Light wind shear
- System stays intact – clouds not blown away
- Strong winds – prevent formation of vortex
5. High humidity
- Latent heat (heat stored when evaporation takes place) is released after
condensation and provides thermal energy
6. Unstable air
- Air must rise so pressure drops
- Rising air cools – condensation – releases latent heat
7. Very low LP and steep pressure gradient
- Rapid convergence – air starts to rise
- Steep pressure gradient – strengthens CF – causes air to rotate
8. Divergence in upper air
- Removes air at higher altitudes and maintains LP on surface
9. No friction
- Winds can reach great speeds – CF remains strong – rotation continues
Storm levels Kilometres per Hour Knots
(kph)
Tropical depression 0 - 60 0 - 33
Group of storms that gets
organised and starts to circulate
